Chronic Total Occlusion (CTO) is an arterial vessel blockage (typically of plaque) that obstructs blood flow. CTO can occur both in coronary and peripheral arteries, and generally results from the same underlying cause—atherosclerosis.
One of the main difficulties in crossing a total occlusion is that the clinician does not know exactly how hard the plaque is until steering a guidewire to the occlusion. If the occlusion is relatively new, the plaque is likely to be soft enough and the guidewire may penetrate the plaque. However, after several weeks or months, the occlusion becomes fibrotic and calcified and the plaque becomes much harder, rendering guidewire crossing of the occlusion difficult if not impossible. Failure to cross the obstruction is the primary failure mode for CTO recanalization.
Another problem is that the CTO blocks contrast agents from flowing through the artery past the obstruction, preventing use of fluoroscopy to guide the guidewire. This increases the risk of perforating or dissecting the vessel, and may possibly increase the risk of tamponade—blood leaking out of the artery around the heart or peripheral organ. Even crossing a partially occluded blood vessel, especially long and curved occlusions, can be difficult and time consuming.
As is well known in anatomy, arteries generally have three coats or layers: an internal or endothelial coat (tunica intima of Kölliker); a middle or muscular coat (tunica media); and an external or connective-tissue coat (tunica adventitia). The two inner coats together are easily separated from the external adventitial layer, and the two inner coats are sometimes referred together as the intimal layer rather than the medial and intimal layers. It is known in the art that during an attempt to get past an occlusion with a guidewire, the guidewire sometimes inadvertently penetrates into the subintimal space between the intimal layer and the adventitial layer of the blood vessel as it attempts to cross the occlusion. Once in the subintimal space, it is very difficult and in many cases impossible to direct the guidewire back into the blood vessel true lumen beyond the occlusion.
However, techniques have been developed for entering the subintimal space on purpose and reentering the true lumen after the occlusion. This so-called subintimal recanalization can be a useful procedure, especially when using drug eluting stents, and is widely used. One of the advantages of subintimal recanalization is that a dissection of the subintimal space is more likely to produce a smooth lumen and improved blood flow than a lumen produced by plowing through calcified plaque. However, technical failure occurs in about 30% of patients undergoing percutaneous intentional extraluminal recanalization, mostly due to the inability to reenter the distal true lumen.
If during percutaneous extraluminal recanalization, the true lumen cannot be reentered with guidewire manipulation, a true lumen reentry device must be used. Currently there are two specially designed reentry devices in the market.
The pioneer reentry catheter (from Medtronic, Santa Rosa, Calif., US) is a 7 Fr. intravascular ultrasound (IVUS) device that is placed in the dissection beyond the occlusion. The IVUS image provides an image of the vessel wall. The catheter is constructed with a monorail lumen for delivery of the device over a 0.014 inch wire, and a second wire lumen through the end of the catheter, which ends in a curved nitinol needle that can retract into the catheter near the distal end. The needle is deployed by sliding it out of a distal port at the side of the catheter just proximal to the IVUS transducer. The IVUS device is used to ultrasonically guide, turn and manipulate the curved needle to arrive at the correct radial orientation for reentry into the true lumen.
The Outback LTD reentry catheter (Cordis, Miami Lakes, Fla., US) is a 6 Fr. catheter with a retractable nitinol curved needle at the distal end. The needle is straight when withdrawn in the catheter. When pushed forward, the needle is restored to its curved shape and can penetrate the medial and intimal layers to reenter the true lumen. The rotational orientation of needle deployment is provided by fluoroscopic guiding markers on the catheter.